Columbine High School

Ed Andrieski/Associated Press

By GINA LAMB | Updated: April 17, 2008

On the morning of April 20, 1999, Eric Harris, 18, and Dylan Klebold, 17, walked into Columbine High School, outside Denver, and shot to death 12 fellow students and a teacher. They then killed themselves. Their actions were the result of a yearlong plot that included plans to blow up the school and kill as many as 500 people.

Ten students, including Mr. Harris and Mr. Klebold, were found dead in the Columbine library. The teacher died of his wounds in a classroom. Two more students were found dead outside the school.

The Columbine rampage was on record as the deadliest school shooting in United States history until a student killed 32 people, and himself, at Virginia Tech on April 16, 2007.

The father of one of the boys was asked some years ago to jot down his life's goals in the memory book for his 20th high school reunion. His answer was succinct, straightforward, and, it seemed, not unrealistically ambitious: ''Raise two good sons.''

Eric Harris and Dylan Klebold, students at Columbine High School, worked last Friday making pizzas. On Monday, they went bowling. And on Tuesday, it seems, they committed mass murder. Nobody had taken the two youths seriously.

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Darion M. Aguilar, who killed two people at a Maryland shopping mall in January, had visited websites devoted to shootings with mass fatalities, and had acknowledged in his journal a need for psychological help.